In the art various proposals have been made to achieve effective printing on substrates by causing a change of color in the substrate on which the printing is to appear.
Various marking components, i.e. activatable pigments, have been proposed, which may be used to mark a substrate upon application of laser energy. As an example, WO 02/01250 discloses the use of oxymetal salts, such as ammonium octamolybdate (AOM), in laser marking.
However, as recognized in the art, laser imageable aqueous compositions based on AOM suffer from giving rise to an opaque coating before activation.
Use of organic soluble amine molybdates in organic solvents to provide transparent coatings suitable for use in color change laser imaging applications is taught in WO 2004/043704. However, organic solvent based compositions, particularly those containing VOCs, are becoming less and less environmentally acceptable and consequently there is a need for water based alternatives that retain the excellent transparency properties exhibited by solvent based systems.
WO 2008/075101 teaches laser imageable alkaline formulations, typically non-aqueous, comprising an off-white dispersion of molybdenum trioxide and sensitizing compound.
EP 1 907 218 teaches water based laser imageable formulations comprising oxygen containing transition metal compounds and additives selected from the groups consisting of organic acids, polyhydroxy compounds and bases. In the case of bases, a stoichiometric excess should be used.
Thus, there is a need within the art for clear laser imageable aqueous compositions.